Synchrotron x-ray studies of single-layer TiSe2 reveal displacements of the Ti and Se atoms as a function of temperature. The measurements, with a high sensitivity of 0.001 Å, show a (2x2) charge density wave (CDW) structure at temperatures below a critical temperature of TC1 = 233 K. The temperature dependence follows a BCS-like second-order mean-field behavior. A fivelayer TiSe2 film also exhibits a CDW transition of the same character but at a lower transition temperature of TC5 = 204 K, which is the same as that for bulk TiSe2. The results demonstrate that lattice distortion is an integral part ofmore » the CDW transition that must also involve renormalization of the electronic structure.« less

The Ho2Ti2O7, Er2Ti2O7 and Yb2Ti2O7 pyrochlores were studied by synchrotron X-ray diffraction to determine whether the (002) peak, forbidden in the pyrochlore space group Fd-3m but observed in single crystal neutron scattering measurements, is present due to a deviation of their pyrochlore structure from Fd-3m symmetry. Synchrotron diffraction measurements on precisely synthesized stoichiometric and non-stoichiometric powders and a crushed floating zone crystal of Ho2Ti2O7 revealed that the (002) reflection is absent in all cases to a sensitivity of approximately one part in 30,000 of the strongest X-ray diffraction peak. This indicates to high sensitivity that the structural space group ofmore » these rare earth titanate pyrochlores is Fd-3m, and that thus the (002) peak observed in the neutron scattering experiments has a non-structural origin. The cell parameters and internal strain for lightly stuffed Ho2+xTi2-xO7 are also presented.« less